Both optical imaging and spectroscopy have been applied to the non-invasive characterization of tissues. Imaging techniques, such as optical coherence tomography (OCT) [1], excel at relaying images of tissue microstructure while spectroscopic methods, such as Raman spectroscopy (RS) [2], are capable of probing the molecular composition of tissue with excellent specificity. The ability of the OCT to perform real-time cross-sectional imaging with micrometer-scale resolution has been utilized for both quantitative and qualitative assessment of tissues in a wide range of applications. For example, quantitative measurements of retinal nerve fiber layer thickness can provide valuable information for glaucoma assessment [3], while qualitative analysis of the esophageal epithelium can identify characteristic features of Barrett's dysplasia [4]. Although visualization of tissue microstructures is often sufficient to characterize tissue type, different structural features can often have a fairly similar appearance in the OCT despite having different underlying molecular makeups [5-7]. This limitation results from the fact that the OCT images are simply maps of reflectivity and do not directly reveal the molecular composition of the sample. The RS, on the other hand, can generate in-elastic scattering spectra with sharp spectral features corresponding to the vibrational modes of biological molecules intrinsic to the sample. The RS has demonstrated the ability to characterize the molecular features of pathology in a number of tissues, including the cervix [8], skin [9], breast [10], and GI tract [11]. In contrast to the OCT, the primary limitation of the RS is that the weak nature of in-elastic scattering precludes rapid spectral imaging over a large spatial area. Clearly, characterization of both the morphological and biochemical composition could compensate for the limitations of both the RS and OCT and allow for a more complete analysis of tissues. For example, the detection of early dental caries has already been identified as a potential application where the mutual benefit of morphological and biochemical characterization OCT and RS can be beneficial [12]. The mutually complementary strengths and limitations of the RS and OCT are well suited for integration into a single instrument for more thorough tissue analysis. The realization of such an instrument allows data collected from the two modalities to augment one another and could advance the biomedical applications of the RS and OCT beyond what is possible with either technique independently.
The most straightforward approach for combination of the RS and OCT into a single instrument includes integrating the sampling optics while maintaining independent detection hardware. To date, the two reports of instruments combining the RS and OCT have pursued the common sample arm approach. The first system combined a time-domain OCT engine using a 1310 nm source and rapid-scanning optical delay reference arm with a 785 nm RS system [13]. This instrument demonstrated the ability of the RS-OCT to perform in vivo analysis and evaluated highly scattering tissues such as the breast and skin. Specifically, the instrument demonstrated the benefits of the RS-OCT by utilizing the OCT to guide Raman spectral acquisition of small (<500 μm) regions of irregular tissue, and utilizing the RS to characterize the biochemical composition of ambiguous structures within an OCT image. A second RS-OCT system combined a Fourier-domain OCT system with an 855 nm broadband source and a spectrometer based detection system (i.e., spectral-domain OCT) with a 633 nm RS system [14]. The advantage of previously reported RS-OCT systems is that the use of independent detection arms allows hardware configurations for each technique to be optimized independently. The drawback, however, is that such configurations require extensive instrumentation that may not be necessary if it are possible to further integrate the two modalities. Since both the RS and OCT can be performed with systems that incorporate a spectrograph and CCD for detection [15, 16], it is possible that a streamlined instrument with a common detection arm can be realized with the appropriate design considerations. The primary challenges in the design of a common-detector RS-OCT system are selection of the appropriate light sources, spectrograph design and selection of appropriate detector architecture.
Therefore, a heretofore unaddressed need exists in the art to address the aforementioned deficiencies and inadequacies.